1. Field of the Invention
The present invention relates to an apparatus and a system for detecting a target such as a person, an animal, an object, etc. intruding into a monitor area.
2. Description of the Related Art
There have conventionally been a camera, a laser device, a monopulse radar device, etc. for detecting any number of targets T intruding into a predetermined monitor area as shown in FIG. 1A. The position of a target T is expressed by, for example, (x, y) in the Cartesian coordinate system (orthogonal coordinate system), and (r, θ) in the polar coordinate system.
However, since the detection device of an optical system such as a laser device, etc. has normally a very narrow beam width, a number of detection devices have to be provided as shown in FIG. 1B to detect a plurality of targets although it is capable of pinpointing a target. In the example shown in FIG. 1B, six laser devices 11 through 16 are provided around the monitor area to detect targets T1 and T2 by radiating laser light among the devices.
Although a detection device provided with a scanning mechanism is used, the detecting capability is considerably reduced by the dirty optical parts due to a spider's web, dust, etc. in an exposure environment especially at an intersection, a crossing, etc. Therefore, frequent maintenance is required. Furthermore, a change in the optical characteristic of a peripheral environment depending on the time can be a serious problem in guaranteeing the performance.
On the other hand, the detection device of a radio system such as a radar device, etc. is advantageous in resistance to environment, but there is the problem in performance because a target is to be localized normally as a point in the polar coordinate system. For example, a monopulse radar device has a relatively wide angle measurement range per device, but only obtains the angle of a single target. A radar device having an array antenna can calculate the angles of a plurality of targets, but is a narrow angle measurement range per device, and requires a large signal processing cost.
There is also a method of measuring only the sight line distance information about a target simply using a plurality of radar devices, solving a set of equations of circles according to the obtained information, and calculating the position of a target in the orthogonal coordinate system. When a target is uniquely localized by the triangulation only according to the distance information, it is necessary to mount a detection device having the angle measurement range of π/2 (90°) at three different points around the monitor area as shown in, for example, FIG. 1C when the monitor area is rectangular. Therefore, at least three detection devices are required.
In the example shown in FIG. 1C, detection devices 21 through 23 are mounted at the tree vertexes of the monitor area, and the positions of the targets T1 and T2 are identified by the triangulation. The targets T1 and T2 are located on the following arcs when viewed from each of the detection devices.                1. detection device 21 T1: arc 24 T2: arc 25        2. detection device 22 T1: arc 26 T2: arc 27        3. detection device 23 T1: arc 28 T2: arc 29        
However, since the angle measurement range for a current monopulse radar device is at most π/6(30°), it is necessary to constitute each detection device by three radar devices to uniquely localize a plurality of targets with the above-mentioned configuration, thereby requiring a total of nine or more radar devices.
The following patent document 1 relates to a method of detecting an object to be monitored using the radar of a spread-spectrum system.                [Patent Document 1] Japanese Published Patent Application No. H9-257919        
However, the above-mentioned conventional target detection apparatus has the following problems.    (1) A detection device of an optical system such as a laser device, etc. is subject to an influence of an ambient environment, and it is difficult to guarantee the performance when it is mounted outdoors. Additionally, it is necessary to mount a number of detection devices to detect a plurality of targets.    (2) There is a rule that one monopulse radar device detects one target. Therefore, to detect a large number of targets and localize each target according to angle information and distance information, it is necessary to mount the number of radar devices equal to the number of targets. In addition, there is a method of arranging each set of three monopulse radar devices at each of the three points around the monitor area, and calculating the position of a target by the triangulation only according to the distance information. However, since the parts constituting a radar device are expensive, it is desired to realize an angle measurement in a wide range using the smallest possible number of devices.
Therefore, to reduce the number of radar devices, for example, as shown in FIG. 1D, the detection devices 31 and 32 each including three radar devices are mounted at two points, and it is checked whether or not targets can be localized with the configuration. The true targets T1 and T2 are located on the following arcs when viewed from each of the detection devices.                1. detection device 31 T1: arc 41 T2: arc 42        2. detection device 32 T1: arc 43 T2: arc 44        
Assuming that the distances from the detection device 31 to T1 and T2 are respectively r11 and r12, and the distances from the detection device 32 to T1 and T2 are respectively r21 and r22, the coordinates of each target is determined by the simple simultaneous equations of circles by pairing with correct measured distance such as T1(r11, r21) and T2(r12, r22), etc. However, the coordinates of the virtual images 51 and 52 are calculated by wrong paring.
Assume that the result as shown in FIG. 1E is obtained by measuring the sight line distance of the targets T1 and T2 in this system. In FIG. 1E, the horizontal axis indicates a distance index, and the vertical axis indicates the power of a baseband signal. The baseband signal in the detection device 31 show two peaks 61 and 62, and the baseband signal in the detection device 32 show two peaks 63 and 64. The distance indexes corresponding to the positions of these peaks indicate the distances to the targets T1 and T2.
At this time, since the peak 63 or 64 can be combined with each of the peaks 61 and 62, there can be two combinations, and the coordinates of a total of four points can be obtained in the monitor area. That is, they are the coordinates of the targets T1 and T2, and the virtual images 51 and 52.
The number of solutions of the simultaneous equations of circles increases in proportion to the squared number of targets. Therefore, with an increasing number of targets for the two detection devices, a higher calculating cost is required to remove detection errors, and there immediately occurs a problem if the number of devices is reduced.    (3) When a large number of targets intrude into a monitor area, it is desired to localize the position of each target as a point in the orthogonal coordinate system or the polar coordinate system to correctly discriminate the targets. However, it is very difficult to perform the process of discriminating a number of targets in a predetermined time using only an autonomous position meter in the conventional detection device in an optical system or a radar system.